166 THE CHROMOSOMES 
method by which the strands are found joined at the 
time when they draw apart, that cross union of the 
threads must have previously taken place. 
If crossing over be supposed to take place between 
two single threads (Fig. 24) all four gametes that 
ultimately result from such a cell will be crossover 
gametes. On the other hand, if crossing over takes 
aaa 
Fic. 47.—Four stages in crossing over, according to ine “typical” 
chiasma type of Janssens. The white rod and the black rod are each 
split depatharibe crossing over takes place only between two of the four 
strands 
place by means of the chiasmatype (Fig. 47) only 
two of the resulting four cells will be crossover 
gametes, the other two being non-crossover gametes. 
Looked at from the point of view of the total 
output, there would be no way in which to tell 
whether one or the other of the above processes has 
taken place; although the formation of a given 
number of crossover gametes involves only half as 
many participating cells in the case of the single 
thread type as in the case of the double thread type. 
1Tf, after the thick threads have split, crossing over involving both 
strands of each chromosome should take place, instead of only one 
strand as in the chiasmatype, sensu strictu, all four gametes that result 
would be crossover gametes. 
